Over the past 20 years, revolutionary changes have occurred in the method and apparatus used for harvesting and curing tobacco. These changes have stemmed largely from the development of bulk curing techniques.
Essentially, the bulk curing method involves packing large quantities of tobacco leaves together in a face to face relationship in a heated, forced air recirculation kiln or barn. The leaves are placed so that their surfaces lie in a vertical plane and the heated air is forced upwards through the bulk pack to cure all of the tobacco within. An example of bulk curing methods is disclosed in Hassler Canadian Patent 751,105.
Various methods and means were developed for making bulk packs and for simplifying and mechanizing tobacco harvesting and handling operations. Initially, bulk packing of the tobacco leaves was accomplished by piercing the leaves onto a rack of tynes. An example of such tobacco rack is disclosed in U.S. Pat. No. 3,143,370 to T. J. Danford et al. Harvesters were adapted to this type of rack, for example, see Long U.S. Pat. No. 3,834,137. Loading and unloading mechanisms for handling bulk packed tobacco were also developed, see Long U.S. Pat. No. 3,095,230.
One of the important steps towards the simplification and mechanization of tobacco harvesting and curing was the development of methods and means for curing in large bins. An example of a tobacco container or bin and the related method of curing is disclosed in Long U.S. Pat. No. 3,935,959. In essence the concept is that the tobacco leaves are distributed into a bin with the leaf faces horizontal but otherwise randomly oriented until the bin is filled. During this loading operation the bin lies on its back. Then the bin is closed and the tobacco leaves are pierced through vertically by retaining rods. The bin is then turned so that the back is upright and the leaf faces are vertical. The formation of the bulk pack is maintained by the now horizontal retaining rods. The bin, in this position, with other bins, is then placed in a forced air recirculation kiln. The bottom and the top of the bin are usually perforated or open to permit the circulation of curing air through the bulk packed tobacco.
In the curing of tobacco leaves in large bins or racks, it is especially important to insure the proper air flow throughout the entire curing cycle. As the warm air moves upwardly through the column of leaves, the leaves gradually give up their water content causing them to shrink. The drying and shrinking of the leaves often tends to allow a passage for the curing air along the other edges of the bins, bypassing other leaves, with the result that some of the tobacco leaves are not fully treated.
Various attempts have been made to control the flow of curing air. U.S. Pat. No. 3,932,946, for example, teaches that gas leakage along the side walls caused by shrinkage during curing may be impeded by a flange placed on the side walls. U.S. Pat. No. 4,259,787 also teaches the use of side wall projections to reduce the leakage along the side walls. While these attempts to control the air flow have met with some success, it has been found that nevertheless, it is desirable to have means to provide better control of the air flow throughout the tobacco bin during the entire curing cycle so as to improve the uniformity of treatment of all leaves in the bin.